Growth features: share buttons, NEW badge, sticky CTA, newsletter cross-promote

- Share buttons on episode and clip cards (Web Share API + clipboard fallback)
- NEW badge on latest episode card
- Sticky call-in CTA bar (appears after hero scrolls out)
- Daily AI Briefing newsletter cross-promote in footer
- Bump cache versions to v=5

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

website/404.html

@@ -15,7 +15,7 @@
   <link rel="icon" type="image/png" sizes="16x16" href="favicon-16.png">
   <link rel="apple-touch-icon" href="apple-touch-icon.png">
 
-  <link rel="stylesheet" href="css/style.css?v=4">
+  <link rel="stylesheet" href="css/style.css?v=5">
   <script defer data-domain="lukeattheroost.com" data-api="/p/event" src="/p/script"></script>
   <script>window.plausible = window.plausible || function() { (window.plausible.q = window.plausible.q || []).push(arguments) }</script>
 </head>